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Laser Safety

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RP Photonics can give helpful advice on laser safety, in particular on various technical aspects and on practical safety guidelines.

Definition: safety of the use of laser devices

German: Lasersicherheit

Categories: lasers, methods

How to cite the article; suggest additional literature

Note: this article is meant to provide some basic information and reasonable guidelines for laser safety, rather than a precise document on official regulations. RP Photonics does not accept any legal responsibility for the correctness of details.

Hazards of Laser Light

Laser beams can be hazardous, particularly for the eye (and sometimes also for the skin), mostly because they can have high optical intensities even after propagation over relatively long distances. Even when the intensity at the entrance of the eye is moderate, laser radiation can be focused by the eye's lens to a small spot on the retina, where it can cause serious permanent damage within fractions of a second – even when the power level is only of the order of a few milliwatts. Damage can result from both thermal and photochemical effects. Laser damage of the eye is not always immediately noticed: it is possible e.g. to burn peripheral regions of the retina, causing blind spots which may be noticed only years later.

Ultraviolet lasers can cause corneal flash burns, a painful condition of the cornea. UV radiation can also cause photokeratitis and cataracts in the eye's lens. (For these reason the XeCl excimer laser has acquired the nickname “cataract machine”.) Mid-infrared lasers, particularly those operating at certain wavelengths with very strong absorption in the cornea (e.g. 3 μm or 10 μm), can also cause painful corneal injuries.

laser safety issues

How much light an eye can tolerate depends on many circumstances: not only the intensity, but also particularly the wavelength and the duration of irradiation (e.g. the pulse duration). There are detailed sets of rules for calculating safe exposure limits (maximum permissible exposure, MPE) for a given situation. Such rules are occasionally revised according to new scientific findings.

Eyes are particularly sensitive, but laser radiation can also cause skin injury. For infrared light, this occurs mainly via thermal effects (thermal skin burns), similar to burning the skin with other means. The penetration depth depends on the wavelength, and for such reasons a laser beam at 1.5 μm wavelength causes more pain on the skin than a 1-μm beam. Whereas such burning should in most cases not have serious long-term consequences, ultraviolet light can in addition induce photochemical reactions. These can lead to changes in the pigmentation, erythema (sunburn), and (most importantly) skin cancer.

Some laser safety issues arise from indirect effects of laser radiation:

Not Only Light is Dangerous

Further issues are not even related to laser beams:

In fact, probably most victims of accidents with lasers have been hurt by such hazards (particularly by electric shocks) rather than by laser radiation.

Particularly Hazardous Situations

The following list of important safety issues can never be complete, but is meant to improve awareness of the multitude of possible hazards:

Often less dangerous are the following cases:

Safety Classes

To give some guidance on adequate handling and required precautions, laser devices are assigned to different safety classes, with class 1 being the least dangerous (containing e.g. lasers with microwatt power levels) and class 4 the most hazardous one. Note that the assignment to a laser safety class depends not only on the laser power, beam quality and laser wavelength, but also on the accessibility of hazardous areas: even a high-power laser may be in safety class 1 when there is no risk that dangerous radiation can leave a fully encapsulated housing.

Details such as the large diameter or the divergence of involved laser beams are largely ignored in such simplified classification schemes. The concept is solely to classify the laser product itself according to some emission limits, rather than evaluating a particular setup containing a laser. The classification is indirectly based on some exposure limits for the eye (see above), but also takes into account a number of worst case assumptions concerning e.g. the distance of persons from the laser aperture, the exposure duration and the possible use of optical instruments. Therefore, the classification tends to overestimate certain risks, and a complete safety assessment has to consider the details of the whole setup and the way it is used.

Table 1: International laser safety classes, with somewhat simplified and approximate descriptions. For details, consult the applicable laser safety standard documents.

Safety classSimplified description
1The accessible laser radiation is not dangerous under reasonable conditions of use.
Examples: 0.2-mW laser diode, fully enclosed 10-W Nd:YAG laser
1MThe accessible laser radiation is not hazardous, provided that no optical instruments are used, which may e.g. focus the radiation.
2The accessible laser radiation is limited to the visible spectral range (400–700 nm) and to 1 mW accessible power. Due to the blink reflex, it is not dangerous for the eye in the case of limited exposure (up to 0.25 s).
Example: some (but not all) laser pointers
2MSame as class 2, but with the additional restriction that no optical instruments may be used. The power may be higher than 1 mW, but the beam diameter in accessible areas is large enough to limit the intensity to levels which are safe for short-time exposure.
3RThe accessible radiation may be dangerous for the eye, but can have at most 5 times the permissible optical power of class 2 (for visible radiation) or class 1 (for other wavelengths).
3BThe accessible radiation may be dangerous for the eye, and under special conditions also for the skin. Diffuse radiation (as e.g. scattered from the some diffuse target) should normally be harmless. Up to 500 mW is permitted in the visible spectral region.
Example: 100-mW continuous-wave frequency-doubled Nd:YAG laser
4The accessible radiation is very dangerous for the eye and for the skin. Even light from diffuse reflections may be hazardous for the eye. The radiation may cause fire or explosions.
Examples: 10-W argon ion laser, 4-kW thin-disk laser in a non-encapsulated setup

Note that there are different classification schemes (e.g. international and American ones), using classes such as 1 to 4 but with somewhat different definitions. (The American system uses classes I, IA, II, IIIA, IIIB and IV similar to the classes 1 to 4 of the international system, but with significant deviations.) Particularly important standards are

The IEC standard has been fully adopted by the European standardization organization as EN 60825-1 and is published in national versions such as DIN EN 60825-1 in Germany. Note that these standards cover much more than only defining safety classes; they also determine the measures to be taken in order to work safely with laser products in such classes. There are also government regulations such as the relatively outdated 21 CFR 1040.10, which is still relevant for the US, although the IEC / EN standard is now also accepted there with some additions.

Generally, it is the duty of the manufacturer of a laser product to classify the product and to equip it accordingly with warning labels. However, the classification may change when a laser product is modified by a user, and the user is then responsible for reclassification.

The Nominal Hazard Zone

Originally, the required safety measures for a given laser setup where basically determined only by the safety class of the laser. As mentioned above, this classification does not reflect details such as beam divergence, which can be very relevant for safety issues: a strongly focused laser beam can be so divergent that within a moderate distance after the focus the intensities fall below the allowable exposure level for the eye. In such situations, one sometimes defines a “Nominal Hazard Zone” (NHZ) within which safe exposure levels may be exceed, in order to apply certain restricting measures to this zone instead of the whole room.

Technical Precautions

Examples of frequently used technical laser safety precautions are:

Non-technical Measures

Technical measures alone are generally not sufficient for keeping safety hazards under control. A number of non-technical measures are therefore very important:

It is also very important to establish a spirit which motivates all staff to take safety issues serious, recognize responsibilities for themselves and for their colleagues, suggest practical solutions, etc.

Laser Safety Regulations

Making laser safety regulations for some production facility is a difficult task, and for a research laboratory it is even harder. The reason is that there are partially conflicting goals:

It is clear that various trade-offs are inevitably involved, e.g. between compactness and suitability for many different circumstances, or between safety and productivity. Giving absolute priority to maximum safety while ignoring productivity and similar practical requirements will not even serve safety, because it increases the risk of regulations being ignored or forgotten. It can be difficult task to analyze existing hazards and to identify the most practical way of dealing with them.

Common Obstacles

Unfortunately, reasonable laser safety regulations are either not in place or (more frequently) routinely ignored in many places such as research and development laboratories. Possible reasons (but not good excuses) are:

Due to such factors, which are difficult (if not impossible) to eliminate altogether, perfect laser safety (making accidents impossible) is probably impossible to reach. However, sensible regulations can greatly diminish the risks without affecting the productivity too severely.

A few guidelines to keep in mind:

  • Thoughtful risk assessment and sensible regulations are required before accidents occur (i.e. before the work begins).
  • Regulations must be practical and convincing, because otherwise they are likely to be breached. Staying on the “safe” side by imposing unrealistic and nonsensical rules on workers will undermine the respect for the regulations, and can thus be very counterproductive.
  • All responsibilities must be clarified for everyone involved. The assignment of a laser safety officer alone (possibly as a scapegoat without sufficient time and powers for enforcement of rules) is not sufficient.
  • Stupid arguments for breaching rules, e.g. of the style “we have always done it like that” or “I know others who also do that”, must be banned.
  • Formal adherence to given rules is not sufficient – operators must stay risk-aware during routine work.

Bibliography

[1]International Electrotechnical Commission (IEC), origin of international laser safety standards: IEC 60825-1 (“Safety of laser products – Part 1: Equipment classification, requirements and user's guide”) and IEC-60825-2 (“Safety of laser products – Part 2: Safety of optical fibre communication systems (OFCS)”), IEC, Geneva
[2]American National Standards Institute (ANSI), http://www.ansi.org/, origin of the American Z-136 safety standard series, in particular the important Z-136.1
[3]Laser Institute of America on laser safety, http://www.lia.org/subscriptions/safety_bulletins
[4]Occupational Safety & Health Administration, U.S. department of labor, “Technical Manual on Laser Hazards”, http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_6.html
[5]J. Krüger and C. Spielmann, “Femtosecond laser technology in use: safety aspects”, http://www3.interscience.wiley.com/cgi-bin/fulltext/117872642/PDFSTART

(Suggest additional literature!)

See also: eye protection, laser beams, laser pointers, lasers, ultraviolet lasers, Spotlight article 2006-12-16, Spotlight article 2007-07-01, Spotlight article 2007-07-06, Spotlight article 2007-12-18

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